Examples:
cartography
card
archaeology
archetype
monarchy
legal
loyal
purse
personal
cinema
kinase
car
carpenter
hippopotamus
Mesopotamia
dative
data
date
...
Old Church Slavonic
сѧ
←
Proto-Slavic
*sę
(inherited)
→
Polish
się
(cognate)
→
Russian
мыться
(derived)
→
Russian
ся
(cognate)